focus/src/core/orp.ts

/**
 * Optimal Recognition Point (ORP) Calculation for Spritz RSVP Reading.
 * Fixates the eye at ~30%-35% into the word where human ocular fixation naturally rests.
 */

const CJK_REGEX = /[一-龥぀-ヿ가-힯]/;

export function isCJKChar(char: string): boolean {
  return CJK_REGEX.test(char);
}

export function isCJKWord(word: string): boolean {
  return CJK_REGEX.test(word);
}

/**
 * Calculates the 0-based index of the ORP character in cleanWord.
 */
export function calculateOrpIndex(cleanWord: string): number {
  const len = cleanWord.length;
  if (len <= 1) return 0;

  if (isCJKWord(cleanWord)) {
    // CJK characters have high semantic density per glyph
    if (len === 2) return 0;
    if (len <= 4) return 1;
    if (len <= 6) return 2;
    return Math.min(len - 1, Math.floor((len - 1) * 0.35));
  }

  // Standard Spritz ORP Table for alphabetic words
  if (len <= 5) return 1;
  if (len <= 9) return 2;
  if (len <= 13) return 3;
  return 4;
}

export interface SplitWord {
  prefix: string;
  pivot: string;
  suffix: string;
  orpIndex: number;
}

/**
 * Splits a token into prefix, pivot (ORP character), and suffix.
 * Handles leading punctuation (e.g. "“", "《", "(") and trailing punctuation.
 */
export function splitOrp(
  coreWord: string,
  leadingPunct: string = '',
  trailingPunct: string = ''
): SplitWord {
  if (!coreWord) {
    return {
      prefix: leadingPunct,
      pivot: '',
      suffix: trailingPunct,
      orpIndex: 0,
    };
  }

  const orpIndex = calculateOrpIndex(coreWord);
  const prefix = leadingPunct + coreWord.slice(0, orpIndex);
  const pivot = coreWord[orpIndex] || '';
  const suffix = coreWord.slice(orpIndex + 1) + trailingPunct;

  return {
    prefix,
    pivot,
    suffix,
    orpIndex,
  };
}